…according to research done by Colorado natives William and Leander Anderegg. After carefully monitoring numerous aspens to measure their physiological processes, the pair concluded that high summer temperatures are the key factor in mortality.

Perhaps most importantly, they found that trees that are damaged once by hot and dry conditions become much more susceptible to future damage due to a process called cavitation fatigue — the breaking of the water pipe vessel that conducts water from roots up to leaves.

“If that gets interrupted the system doesn’t work any more … if the tree goes through an episode of cavitation, severe water stress, the next time they undergo water stress it happens more easily,” Worrall said, explaining that the Anderegg’s study provided a lot of physiological evidence for how drought affects the trees…

Acute episodes of drought and heat like in 2002 and 2012 could be the tipping point that pushes some aspens over the brink.